Despite the clear importance of language in our lives, the neurobiological underpinnings of our ability to quickly and effectively learn new words and their meanings are poorly understood. Conventional studies typically focus on slow long-term processes accompanying language learning. Whereas language acquisition is indeed a laborious process, especially in adulthood, recent studies suggest that neurophysiological signatures of learning can be near-instantaneous. This raises key questions: is rapid functional learning of words underpinned by equally rapid micro-structural remodelling of the brain? If so, what is the neuroanatomical profile of such changes and can their causal role be established? We address these challenges by using a microstructurally sensitive diffusion MRI technique called diffusion kurtosis imaging (DKI) to identify cytoarchitectural remodelling after mere minutes of word learning. We used a virtual-reality training protocol, in which the learning efficiency in acquiring novel nouns and verbs was modulated by M1 TMS. Our findings show that a single learning session caused rapid plasticity in a number of key brain regions related to learning and language processing, including the hippocampus, anterior temporal cortex, and angular gyrus. Moreover, we demonstrate that the motor cortex is causally involved in the encoding of novel language, and not just retrieval, as was previously shown. Indeed, TMS-induced changes in M1 excitability predicted plastic reorganisation in high-level lexico-semantic regions, both immediately and after an overnight consolidation stage. In sum, our study reveals some of the earliest microstructural signatures of regional brain plasticity potentially indicating its causal relevance for fast word acquisition.